The present invention relates to a process for conditioning waste, constituted by radioactive metal particles, such as fines obtained during the dissolving of irradiated fuel elements and dust obtained during the mechanical decanning and/or cutting up operations of irradiated fuel elements.
In installations for the reprocessing of irradiated nuclear fuel elements, the conventional practice consists of firstly subjecting the fuel elements to a preparatory mechanical treatment carried out e.g. by cutting up or shearing, with a view to facilitating the subsequent dissolving of the fuel in a nitric acid solution. During this operation, it is difficult to avoid the formation of dust and such radioactive metallic dust is largely insoluble in the solutions used for the reprocessing, so that they must be recovered and undergo conditioning. In the same way, during the dissolving of fuel elements, certain metal particles are not attacked, because they are insoluble in nitric acid solution and constitute what is generally called "dissolving fines". The latter are essentially constituted by ruthenium, rhodium, palladium, molybdenum and to a lesser extent by zirconium, niobium, technetium, uranium and plutonium. For example, the attached table gives the nature and composition of the dissolving and shearing fines from light water reactors and fast neutron reactors.
These metal particles form highly radioactive waste, which is difficult to valorize on a short term basis, despite the presence of a significant quantity of metals in the platinum group.
In addition, it is necessary to treat the waste in order to ensure that it is stored in the long term under satisfactory safety conditions.
In view of the fact that the quantity and dimensions of these insoluble radioactive particles increase with the irradiation level, the problem of processing such waste increases in importance with the development of light water reactors and fast neutron reactors, whose fuel elements are subject to high specific burn-ups.
Thus, it is estimated that the processing of one ton of uranium from light water reactor fuels gives approximately 3.5 kg of dissolving fines and that a ton of oxide from fast neutron reactor fuel elements gives 8 to 13 kg of dissolving fines.
Moreover, on taking as an order of magnitude, a reprocessing plant having a capacity of 800 t/year for light water reactor fuels, it would be necessary to process 2800 kg of such fines every year, whilst in the case of a reprocessing plant with a capacity of 150 t/year for fast neutron reactor fuels, there would be 1200 kg of fines per year. Moreover, it is necessary to add to these figures the dust obtained during the shearing and cutting up of the fuels. In the case of light water reactor fuels, the can is made from zircalloy and generally approximately 3 kg/t of shearing fines are produced. In the case of fast neutron reactor fuel elements for which the can is generally made from stainless steel, these shearing fines represent approximately 1 kg/t of uranium.
However, the processing of dissolving and shearing fines causes certain problems, due to their high thermal power linked with their high radioactivity, and in certain cases due to their pyrophoric character, due to the presence of fine zirconium particles resulting from the shearing of light water reactor fuel element cans.
Moreover, it is preferable to process these dissolving and shearing fines in the first stages of the fuel reprocessing process in order to prevent blockages to the pipes because, as the particles have high specific gravities, there is a tendency for them to be deposited in the calm areas of the installation. This also applies in connection with the prevention of local overheating, which causes premature damage to the containers, as well as the decomposition of organic solvents by radiolysis.
Consideration is also given to the separation and recovery of these fines at the outlet from the dissolving installation, so that they can be treated with a view to their conditioning.
Hitherto, for the conditioning of radioactive products, use has been made of various processes, the most important of which involves the coating of the waste in cement, or the vitrification thereof. However, these known processes are limited, when it is a question of conditioning radioactive waste constituted by dissolving or shearing fines. Thus, in the case of cements, the high specific power of the fines is prejudicial to the mechanical behavior of the coating material. Moreover, there are risks of radiolysis of the constitution water of the cement.
The incorporation of these particles into a glass is only possible after the irradiated fuel has been cooled for an adequate time. This serves to prevent the formation within the material of hot points, which favor a heterogeneous crystallization development and incipient cracks. This is why the process, which can be used for conditioning fines from PWR fuels, which are generally reprocessed after several years cooling, is not suitable for conditioning fines from fast neutron reactor fuel, which is generally reprocessed relatively rapidly after unloading from the reactor.
Consideration has also been given to the conditioning of the radioactive waste constituted by particles of oxides or glasses obtained from the solutions, by using metal matrixes, as described in French Pat. No. 2 387 093 and British Pat. No. 1 446 016. However, the products obtained as a result of these processes have a heterogeneous structure, the radioactive glass or oxide particles being dispersed in the metal matrix. Moreover, the process described in French Pat. No. 2 387 093 implies the preparation of a finely divided powder from a solution of radioactive waste containing a salt of the metal forming the matrix, following by a hot compression stage.
Thus, this process cannot be used for processing dissolving fines, because it leads to the formation of a ceramic-metal, which has the same disadvantages as glass from the thermal standpoint. Moreover, the mixture of very high-energy fines in an oxide, which is a poor heat conductor, leads to significant temperature rises in the mixture, to an agglomeration of the mixture and to the impossibility of obtaining a fine powder for fritting.
In the same way, the process of British Pat. No. 1 446 016 is not suitable for processing dissolving fines because, in view of the very small dimensions of such fines, it is impossible to obtain a homogeneous dispersion thereof in the metal matrix by pouring the same into a container containing the dissolving fines. Thus, the products obtained would not have satisfactory characteristics for long-term storage.